A couple of reasons spring to mind:
Use of the phone clients (iPhone, Android and soon to be Symbian)
Higher quality
Also while the P2P feature exists it isn't actually enabled yet (according to wikipedia) but I can imagine if/when it is, subscribers will be able to opt out of the service.
Obviously, if you don't think it's worth it, you don't have to subscribe, but to many people it is. And I presume as it grows in popularity the music collection on there will expand to provide some of those "mega bands" you mention. (Though quite why anyone would listen to Metallica is beyond me. Yawn.)
Personally I'd pay for this just for the ability to listen instantly on my phone and discover new artists as they are suggested to me while I'm out and about instead of having to pay for a track I find I don't like or go the illegal route and hunt around for a download and then wait for it to complete.